1. Field of the Invention
The present invention relates to a photoelectric encoder having a detector that is relatively displaceable to a scale having a predetermined pattern formed thereon, and in particular to a photoelectric encoder capable of measuring a wide detection range using a simple and small-sized optical system and light-receiving system.
2. Description of Related Art
To decrease the sensitivity to stains on a scale in a photoelectric encoder, for example, an incremental type photoelectric linear encoder, it is necessary to widen the detection range on the scale. However, if the detection range is widened, there arises a problem in that the optical system in the inner part of the detector is made large in size.
Also, as has been described in FIG. 1 of Japanese Published Unexamined Patent Application No. Hei-7-286861 (Patent Document 1), a so-called absolute-type encoder has been known, which obtains the absolute position by detecting a pseudorandom pattern disposed on the main scale.
For example, a pattern called an “M-sequence code,” which is described in FIG. 1 of J T M Stevenson et al. “Absolute Position Measurement Using Optical Detection of Coded Patterns,” J. Phys, E: Sci. Instrum. 21(1988) 1140-1145 (Non-patent Document 1) may be used as the pseudorandom pattern.
In the absolute type encoder, a part of the pattern disposed on a scale is detected, and it is determined by comparison with which part of the entirety of the pseudorandom pattern the pattern is made coincident. In detail, as has been described in FIG. 1 and FIG. 3(A) of Japanese Published Unexamined Patent Application No. Hei-8-29200 (Patent Document 2), correlation coefficients between the detection pattern and the pseudorandom pattern are calculated, and a part where the correlation coefficient is made highest is made into the absolute position.
With a method for detecting the absolute position based on such a pseudorandom pattern, if the length for which the absolute position (that is, the absolute detection range) can be detected is attempted to be lengthened, the periodic cycle of the pseudorandom pattern is lengthened, and it is necessary to detect a long pattern to obtain a favorable correlation peak. However, there is a problem in that, since an optical system having a wide field of view is required to detect a long pattern and a large lens is used, the optical system is made large.
As shown in FIG. 1 and FIG. 2, Japanese Published Unexamined Patent Application No. 2004-317503 (Patent Document 3) describes that, as means to improve the above-described problem, the lens array imaging optical systems 3 and 4 are double stacked and used. With this method, after patterns 21 and 22 (O in FIG. 2) on the scale 2 are imaged as an intermediate image O′, it is imaged as a detection image O″ on the light-receiving element 19. In the drawings, reference numeral 1 denotes a scanning unit, 10 denotes a printed circuit board, 11 denotes a light source, L denotes a light beam, 12 denotes a side wall, 13 denotes a condenser lens, 15 denotes a glass plate, A denotes an optical axis, B denotes a light flux, and reference numerals 31a through c and 41a through c denote lenses.
However, in Patent Document 3, since the lens array imaging optical systems are double stacked, it is necessary to accurately set the positional relationship of both the optical systems in order to obtain the final detection image. This becomes a large technical problem in practically constructing the optical system.
Further, in Japanese Published Unexamined Patent Application No. Hei-9-229717 (Patent Document 4), absolute positions at a plurality of detection points are detected, and checked whether difference between detected position data and distance between detection points is coincident or not, to obtain higher reliability of detecting absolute position.
However, both in Patent Documents 4, wide range pattern, which can detect 2N−1 bit in the M-sequence code generated by an N-stage shift register, for example, is necessary to calculate at least absolute position.